I've been sitting on this for a while, mostly because I'm not set up to do video, and people seem to prefer that to photos, but I finally gave in and just shot something off quickly with my phone gaffed to a tripod:

This is a fairly tricky knot, and finicky even once you know it, but in certain situations it is invaluable, most especially this one:

You might ask "but why can't I just do half hitches around the leg, like everybody does?". And the answer is that, if you really load that, from the standing line end, it tends to pull on just the first wrap, and cinch down -- it's not a real column tie. You'll notice that the tutorial I linked that image from then goes on to run the line back up to the harness, which is the usual way to avoid the issue; but with the RSB, you don't have to go back, you could end your rope at the cuff, or do another cuff further down the leg.

When I demonstrated this a while back at the last CT Grue, someone asked me why not just use this all the time instead of the normal Somerville Bowline -- the main reasons are that it's not as quick and easy to tie, and that it's easy to wind up with it too tight, or to wind up pulling unevenly on the first wrap if you're not careful how you lay it up. However, one reason you might want to use the Reverse SB even at the beginning of your line, is if you're tying on someone who's struggling; more about that here.